Method and system of determining motion in a region-of-interest directly and independently of k-space trajectory

ABSTRACT

A system and method for determining motion in a region-of-interest directly from MR data acquired from the region-of-interest and independently of k-space trajectory are disclosed.

BACKGROUND OF THE INVENTION

The present invention relates generally to MR imaging and, moreparticularly, to a method and system of determining motion in aregion-of-interest directly from MR data acquired from theregion-of-interest independent of k-space trajectory of a k-spacefilling scheme carried out to sample the region-of-interest.

When a substance such as human tissue is subjected to a uniform magneticfield (polarizing field B₀), the individual magnetic moments of thespins in the tissue attempt to align with this polarizing field, butprecess about it in random order at their characteristic Larmorfrequency. If the substance, or tissue, is subjected to a magnetic field(excitation field B₁) which is in the x-y plane and which is near theLarmor frequency, the net aligned moment, or “longitudinalmagnetization”, M_(Z), may be rotated, or “tipped”, into the x-y planeto produce a net transverse magnetic moment M_(t). A signal is emittedby the excited spins after the excitation signal B₁ is terminated andthis signal may be received and processed to form an image.

When utilizing these signals to produce images, magnetic field gradients(G_(x), G_(y), and G_(z)) are employed. Typically, the region to beimaged is scanned by a sequence of measurement cycles in which thesegradients vary according to the particular localization method beingused. The resulting set of received NMR signals are digitized andprocessed to reconstruct the image using one of many well knownreconstruction techniques.

Any type of subject motion—cardiac, respiratory, or other bodymotion—during MRI may introduce image artifacts that affect the qualityof an image and, ultimately, diagnostic utility of the scan. A number oftechniques have been developed to reduce motion artifacts in MRI. Oneclass of techniques synchronizes data acquisition with motion usingbreath-holding, respiratory triggering, and/or cardiac gating to“freeze” subject motion. Another class of techniques corrects dataacquired in the presence of motion. Despite the use of these techniques,however, motion remains one of the primary impediments to diagnosticimage quality.

Most known motion artifact reduction techniques require the ability tomeasure subject motion during the scan, either by external physicaltools (e.g. respiratory bellows, electrocardiogram (ECG), pulseoximetry, and the like) or by interleaving extra gradient pulses intothe pulse sequence (e.g. navigator echoes)—both of which imposeadditional time, cost, and complexity to the scan and scan system.Furthermore, external physical measurements are typically indirect,error-prone measures of motion that may not accurately reflect truemotion in the region-of-interest. For example, respiratory bellowsmeasure only the anterior-posterior component of breathing motion at theabdominal surface, while the ECG measures electrical rather thanmechanical cardiac activity. While breath-holding can be used tominimize respiratory motion artifacts and does not require the abilityto measure motion, it limits image quality, suffers from drift andposition inconsistencies, and is often impractical in severely ill orpediatric subjects.

Other known motion assessment techniques are k-space trajectory limited.That is, one known technique determines motion in a region-of-interestdirectly from MR imaging data acquired from the region-of-interest. Thatis, this technique determines motion from spatially encoded data, ratherthan data acquired without spatial encoding. In this regard, thistechnique is severely limited in its applications. Simply, the techniqueis applicable only with spiral and radial sampling. As such, thetechnique cannot be used for MR scans in which resonance is sampled andk-space filled using non-spiral or non-radial k-space trajectories. Asthere are a number of MR imaging techniques that do not rely upon aspiral or radial k-space trajectory, this known technique, and othersthat are k-space trajectory limited, is frequently inapplicable.

Other proposed “self-navigated” techniques sample additional echoes thatare generated without phase encoding. The data associated with theseadditional and fully-sampled echoes is then analyzed to determine motionin a region-of-interest. Since “extra” echoes must be induced and thensampled, such a technique can significantly lengthen scan time andincrease the memory and processing requirements of an MR scanner.

It would therefore be desirable to have a system and method capable ofmore direct, efficient, and accurate measuring of motion in aregion-of-interest that is independent of k-space trajectory such thatmotion artifact reduction techniques may be applied with greater successin many MR applications and without increasing scan time.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is directed to a method and apparatus of assessingmotion in a region-of-interest directly from MR data acquired from theregion-of-interest independent of the k-space trajectory used to fillk-space that overcomes the aforementioned drawbacks.

The present invention is directed to the extraction of motioninformation within a region-of-interest directly from raw k-space datawithout implementation of separate physiological motion measuringsystems. Moreover, by employing a direct extraction of motioninformation from k-space, the present invention provides a measurementof motion in the region-of-interest that is more accurate when comparedto indirect techniques of assessing motion, i.e. ECGs. Additionally, thepresent invention is capable of ascertaining magnitude as well as phaseinformation with respect to motion in a region-of-interest. In thisregard, the present invention may be applied to not only improvingsynchronization of MR data in a gated acquisition, but also used inslice tracking and phase error correction. Furthermore, not only is thepresent invention k-space trajectory independent, it may also beimplemented to assess multiple types of motion in a region-of-interest,such as respiratory, cardiac, and other anatomical induced motion.Moreover, the present invention is applicable with single coil andmulti-coil receivers and, as such, can be used to differentiate locationand direction of motion in an excited region based on coil sensitivityto the excited region. An understanding of location and direction ofmotion can further enable accurate assessment of motion in theregion-of-interest for acquisition synchronization, slice tracking, anderror correction.

The present invention is particularly applicable to body imaging scanswhere respiratory motion can severely limit image quality, especially insubjects who cannot hold their breath for extended periods of time. Thepresent invention may also be applicable in cardiac imaging. In thisregard, a separate ECG acquisition can be avoided and motion informationderived directly from MR data can be used as a cardiac gating triggerfor CINE imaging and other cardiac protocols. Detected respiratorymotion of the heart could also be used to synchronize cardiac imagingwith breathing or to perform slice tracking, particularly in coronaryartery imaging applications, which are vulnerable to breathing motion.

Therefore, in accordance with one aspect of the invention, an MRIapparatus is disclosed and includes a magnetic resonance imaging (MRI)system having a plurality of gradient coils positioned about a bore of amagnet to impress a polarizing magnetic field and an RF transceiversystem and an RF switch controlled by a pulse module to transmit RFsignals to an RF coil assembly to acquire MR images. A computer isprogrammed to sample MR signals from a region-of-interest having motiontherein with a given k-space trajectory to fill k-space and determinemotion in the region-of-interest directly from sampled MR data acquiredfrom the MR signals independent of the given k-space trajectory used tofill k-space and fill k-space with a k-space filling scheme that fillsan origin of k-space at least once every repetition internal of a pulsesequence.

In accordance with another aspect, the present invention includes amethod of MR imaging. The method includes sampling MR data over aplurality of repetition time intervals for a central region of k-spacefilled, using a given k-space filling trajectory, with MR data acquiredfrom the region-of-interest having motion therein. The method alsoincludes the steps of monitoring motion-induced modulation in the MRdata for the central region of k-space over the plurality of repetitiontime intervals and determining motion in the region-of-interest from themotion-induced modulation independent of the given k-space fillingtrajectory.

According to another aspect, the present invention may be found in acomputer readable storage medium having a computer program storedthereon to assess motion in a region-of-interest. The computer programincludes a set of instructions that when executed by a computer causesthe computer to sample a central region of k-space each repetition timeinterval of a pulse sequence applied to acquire MR data from theregion-of-interest. The computer is then caused to measure modulation ofMR data in the central region over several repetition time intervals.The computer is also caused to determine motion in theregion-of-interest based on differences in magnitude and phase measuredin the MR data over the several repetition time intervals independent ofk-space trajectory used to sample the region-of-interest.

In accordance with yet another aspect, the present invention includes amethod of MR imaging that includes acquiring a first set ofnon-spatially encoded MR data from a region-of-interest prior toapplication of spatially encoding gradients and acquiring a second setof non-spatially encoded MR data from the region-of-interest afterapplication of rewinder gradients. Motion in the region-of-interest isthen determined from the first and the second set of non-spatiallyencoded MR data.

Various other features and advantages of the present invention will bemade apparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate one preferred embodiment presently contemplatedfor carrying out the invention.

In the drawings:

FIG. 1 is a schematic block diagram of an MR imaging system for use withthe present invention.

FIG. 2 is a flow chart setting forth the steps of a motion-assessmentand application technique in accordance with the present invention.

FIG. 3 is a pulse sequence diagram incorporating the present inventionwith a radial center-out k-space trajectory.

FIG. 4 is a pulse sequence diagram incorporating the present inventionwith a Cartesian SSFP k-space trajectory.

FIG. 5 is an image illustrating k-space traversal for the radialcenter-out pulse sequence of FIG. 3.

FIG. 6 is an image illustrating k-space traversal for the Cartesian SSFPpulse sequence of FIG. 4.

FIG. 7 is a graph that illustrates a comparison of k-space origin datafor each channel of an eight channel cardiac coil during an axialphantom scan with no table motion.

FIG. 8 is a graph comparing k-space origin data for each channel of aneight channel cardiac coil during an axial phantom scan with periodictable motion.

FIG. 9 is a graph comparing k-space origin data and respiratory bellowsdata from an axial liver scan in a free-breathing subject.

FIG. 10 is a graph illustrating a comparison of k-space origin data andrespiratory bellows data during coronal liver imaging in afree-breathing subject.

FIG. 11 is a graph illustrating a comparison of k-space origin data andrespiratory bellows data during sagittal liver imaging in afree-breathing subject.

FIG. 12 compares k-space origin data extracted from an axial cardiacscan in a free-breathing subject.

FIG. 13 is a schematic representation of a spiral k-space trajectory.

FIG. 14 is a schematic of a centric k-space trajectory.

FIG. 15 is a schematic of a reverse-centric k-space trajectory.

FIG. 16 is a schematic representation of a center-out k-spacetrajectory.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the major components of a preferred magneticresonance imaging (MRI) system 10 incorporating the present inventionare shown. The operation of the system is controlled from an operatorconsole 12 which includes a keyboard or other input device 13, a controlpanel 14, and a display screen 16. The console 12 communicates through alink 18 with a separate computer system 20 that enables an operator tocontrol the production and display of images on the display screen 16.The computer system 20 includes a number of modules which communicatewith each other through a backplane 20 a. These include an imageprocessor module 22, a CPU module 24 and a memory module 26, known inthe art as a frame buffer for storing image data arrays. The computersystem 20 is linked to disk storage 28 and tape drive 30 for storage ofimage data and programs, and communicates with a separate system control32 through a high speed serial link 34. The input device 13 can includea mouse, joystick, keyboard, track ball, touch activated screen, lightwand, voice control, or any similar or equivalent input device, and maybe used for interactive geometry prescription.

The system control 32 includes a set of modules connected together by abackplane 32 a. These include a CPU module 36 and a pulse generatormodule 38 which connects to the operator console 12 through a seriallink 40. It is through link 40 that the system control 32 receivescommands from the operator to indicate the scan sequence that is to beperformed. The pulse generator module 38 operates the system componentsto carry out the desired scan sequence and produces data which indicatesthe timing, strength and shape of the RF pulses produced, and the timingand length of the data acquisition window. The pulse generator module 38connects to a set of gradient amplifiers 42, to indicate the timing andshape of the gradient pulses that are produced during the scan. Thepulse generator module 38 can also receive patient data from aphysiological acquisition controller 44 that receives signals from anumber of different sensors connected to the patient, such as ECGsignals from electrodes attached to the patient. And finally, the pulsegenerator module 38 connects to a scan room interface circuit 46 whichreceives signals from various sensors associated with the condition ofthe patient and the magnet system. It is also through the scan roominterface circuit 46 that a patient positioning system 48 receivescommands to move the patient to the desired position for the scan.

The gradient waveforms produced by the pulse generator module 38 areapplied to the gradient amplifier system 42 having Gx, Gy, and Gzamplifiers. Each gradient amplifier excites a corresponding physicalgradient coil in a gradient coil assembly generally designated 50 toproduce the magnetic field gradients used for spatially encodingacquired signals. The gradient coil assembly 50 forms part of a magnetassembly 52 which includes a polarizing magnet 54 and a whole-body RFcoil 56. A transceiver module 58 in the system control 32 producespulses which are amplified by an RF amplifier 60 and coupled to the RFcoil 56 by a transmit/receive switch 62. The resulting signals emittedby the excited nuclei in the patient may be sensed by the same RF coil56 and coupled through the transmit/receive switch 62 to a preamplifier64. The amplified MR signals are demodulated, filtered, and digitized inthe receiver section of the transceiver 58. The transmit/receive switch62 is controlled by a signal from the pulse generator module 38 toelectrically connect the RF amplifier 60 to the coil 56 during thetransmit mode and to connect the preamplifier 64 to the coil 56 duringthe receive mode. The transmit/receive switch 62 can also enable aseparate RF coil (for example, a surface coil) to be used in either thetransmit or receive mode.

The MR signals picked up by the RF coil 56 are digitized by thetransceiver module 58 and transferred to a memory module 66 in thesystem control 32. A scan is complete when an array of raw k-space datahas been acquired in the memory module 66. This raw k-space data isrearranged into separate k-space data arrays for each image to bereconstructed, and each of these is input to an array processor 68 whichoperates to Fourier transform the data into an array of image data. Thisimage data is conveyed through the serial link 34 to the computer system20 where it is stored in memory, such as disk storage 28. In response tocommands received from the operator console 12, this image data may bearchived in long term storage, such as on the tape drive 30, or it maybe further processed by the image processor 22 and conveyed to theoperator console 12 and presented on the display 16.

The present invention is directed a technique of assessing of motion ina region-of-interest directly from MR data acquired from theregion-of-interest and independently of k-space trajectory of a k-spacefilling strategy that may be implemented on the MRI apparatusillustrated in FIG. 1, or equivalent thereof.

The present invention determines motion in a region-of-interest directlyfrom MR data acquired from the region-of-interest without additionalphysiological motion measuring devices or additional gradient pulses.This extracted data may then be used in a number of motion artifactreduction techniques predicated upon knowledge of the amount and type ofmotion in the region-of-interest including, but not limited tosynchronization of image acquisition with motion or the correction ofdata acquired in the presence of motion. The present invention isapplicable with stationary and moving table MRI, including incrementaland continuous moving table MRI.

Most pulse sequences excite, encode, and readout data from aregion-of-interest with a k-space trajectory that passes through theorigin of k-space at least once every repetition time interval. Giventhat the data at the k-space origin represents the integrated intensityof all transverse magnetization in an excited region, area, orvolume-of-interest, any change or modulation experienced in the MR dataof the k-space origin over time can be attributed to motion-inducedchanges in magnetization, assuming steady-state conditions. For example,in cardiac imaging, changes in myocardial wall thickness and bloodvolume over the cardiac cycle cause periodic fluctuations in transversemagnetization. In body imaging, changes in the position of the lung,diaphragm, and liver due to breathing can cause in-plane and/orthrough-plane movement that modulates magnetization. These effects canbe assessed directly from the MR data in light of magnitude and phasemodulation caused to the MR data at the k-space origin independently ofk-space trajectory using a “self-navigated” technique described herein.As a “self-navigated” technique, the present invention does not requireadditional gradient pulses to extract motion data from aregion-of-interest.

Referring now to FIG. 2, a flow chart setting forth one embodiment ofthe present invention is shown. Process 70 begins at 72 with user inputof scan parameters defining a pulse sequence for data acquisition of aregion-of-interest in a conventional manner. Following setting up theparticulars of the scan, one or more points about the k-space origin areselected for monitoring of motion-induced fluctuations 74. The one ormore points, which are preferably centered about and include the k-spaceorigin, are sampled over several repetition time intervals of the pulsesequence 76. By sampling the selected points of k-space over severalcycles, fluctuations in the MR data corresponding thereto can be readilyascertained at 78. In a preferred embodiment, fluctuations in both themagnitude and the phase of the sampled MR data will be measured and usedto assess the scope and extent of motion in the region-of-interest 80.

The assessment of the motion in the region-of-interest can be applied ina number of ways 82. The magnitude may be used to define cardiac and/orrespiratory cycles. With an understanding to the cardiac and/orrespiratory cycles, the motion information determined at 80 may be usedto synchronize a gated MR data acquisition 84 to improve signal-to-noiseand contrast-to-noise. In addition, the magnitude of motion-inducedfluctuations in the data for the k-space origin can also be used tocarry out slice tracking 86. Additionally, the phase of motion-inducedfluctuations can be used for correcting phase errors 88 in acquired MRdata to reduce image artifacts and improve image quality, whereupon theprocess ends at 89. One skilled in the art will appreciate thatsynchronization, slice tracking, and phase error correction are justthree motion related processes to which the motion informationdetermined from the region-of-interest may be applicable and that iscontemplated that the present invention may be applicable with othermotion artifact reduction techniques.

It is contemplated that synchronization, slice tracking, and phase errorcorrection can be carried out prospectively or retrospectively. That is,it is contemplated that a fast scout scan can be carried out to acquireMR data from a region-of-interest. The MR data can then be assessed, asdescribed herein, to determine motion in the region-of-interest and themotion information learned can be used to define the parameters of asubsequent and full imaging scan. On the other hand, a full imaging scanbe carried out and data in or about the origin of k-space can beaccessed prior to image reconstruction to determine motion in theregion-of-interest. From the motion information, data may be realignedrelative to determined motion in the region-of-interest prior to imagereconstruction so as to reduce ghosting and other artifacts in thereconstructed image.

Referring again to FIG. 2, the present invention is applicable withsingle coil and multi-coil receivers. For a multiple coil assembly, eachcoil has a sensitivity to a region-of-interest that varies between thecoils based on the spatial proximity of each coil to theregion-of-interest. Further, as each coil separately samples signal fromthe region-of-interest and the sampled data is used to fill acorresponding k-space matrix for each coil, the present inventioncontemplates that the data for selected points of k-space can becombined at 90 from the k-space matrices of the multiple coils to assessmotion in the region-of-interest 78. That is, fluctuations in magnitudeand phase of the k-space data is determined from a composite set ofk-space data derived from k-space data acquired by all the coils of themulti-coil assembly, e.g. phased array architecture. On the other hand,it is contemplated that the k-space data from the multiple k-spacematrices can be compared to one another 92 to determine a coil mostsensitive to motion in the region-of-interest 94. The motion informationderived from the k-space of the coil most sensitive to motion in theregion-of-interest is then used to assess motion in theregion-of-interest and subsequent application in motion artifactreduction processes. The coil most sensitive to motion in theregion-of-interest may be determined from which k-space origin has thegreatest fluctuations in magnitude and/or phase of corresponding MRdata.

Referring now to FIG. 3, a pulse sequence diagram illustratesapplication of the present invention with acquisition of MR data from aregion-of-interest using a radial center-out k-space filling scheme.Pulse sequence 96 is defined by a slice select gradient 98 played on inthe presence of an excitation pulse 99, a phase encoding gradient 100,and a frequency encoding gradient 102. Arrows 104 indicate the instantat which the origin of k-space is traversed. In this regard, traversalof the origin of k-space preferably occurs prior to application ofspatially encoding gradients 100 and 102 and again after application ofrewinder gradients 106 and 108. Accordingly, in the illustrated pulsesequence, the origin of k-space is sampled twice each repetition timeinterval. Specifically, an A/D converter of MRI system 10, FIG. 1, iscaused to be turned ON by the system control 32, FIG. 1, at moments 104for the sampling of k-space origin. The A/D converter will remain ON 110during readout, turn OFF 112 after readout, await application ofrewinder gradients 106 and 108, and the briefly return to an ON state at114. By sampling the origin of k-space before application of spatiallyencoding gradients and after rewinder gradients, it is possible tocompare the data acquired at both instances to gain insight into theextent of motion during the acquisition of imaging data, i.e. duringreadout. Additionally, as will be described, the acquisition of themotion data in addition to the imaging data allows for dual purpose RFcoils.

As described herein, the present invention is not dependent upon a givenk-space trajectory. FIG. 3 illustrated a radial center-out k-spacetrajectory to which the present invention is applicable. FIG. 4illustrates a Cartesian steady-state free precession (SSFP) pulsesequence 116. Pulse sequence 116 is defined by a slice select gradient118 played out in the presence of an excitation pulse 120 to excite andencode a region-of-interest. The pulse sequence also includes a phaseencoding pulse 122 and a frequency encoding or readout pulse 124. Again,arrows 126 indicate the instant of k-space origin traversal. The A/Dconverter samples the region-of-interest before application of thespatially encoding gradients at 128, during readout at 130, and afterapplication of rewinder gradients 132, 134 at 136.

One skilled in the art will appreciate that neither pulse sequence 96nor pulse sequence 116 employs additional gradient pulses to encodemotion in the region-of-interest or excites additional echoes. In thisregard, by appropriately timing of the A/D converter to sample theorigin of k-space, motion data can be captured without requiringapplication of additionally time consuming gradients or excitingnon-imaging echoes. As described above, the data corresponding to theorigin of k-space can then be monitored for fluctuations in magnitudeand/or phase to assess motion in the region-of-interest without a timepenalty.

Referring now to FIGS. 5-6, k-space traversal corresponding to theradial center-out pulse sequence 96, FIG. 3, and the Cartesian SSFPpulse sequence 116, FIG. 4, are shown, respectively. Solid lines depictreadout trajectories, while dashed lines depict prewinder and/orrewinder gradients. As illustrated, for both pulse sequences, thek-space trajectory starts and ends at the k-space origin, at which pointmotion encoded MR data can be acquired.

As described herein, the present invention is applicable with singlecoil as well as multi-coil receivers. In this regard, the need for a“motion-only” coil designed to acquire motion data separate from an“imaging” data coil is eliminated. Simply, a coil used to acquire motiondata is also used to acquire imaging data. Shown in FIG. 7 is a graphillustrating a comparison of k-space origin data recorded from eachchannel of an eight channel cardiac coil during an axial phantom scanwith no table motion. As illustrated, fluctuations in the k-space origindata are relatively minor and, as such, motion in the region-of-interestis at a relatively steady-state over course of a scan.

Conversely, and illustrated in FIG. 8, regular fluctuations in signalamplitude caused by through-plane phantom motion are visible for k-spaceorigin data acquired with an eight channel cardiac coil during an axialphantom scan with periodic table motion (ten mm excursion every fiveseconds). Further, each channel (or coil) shows a different sensitivityto motion depending on its relative proximity to the excitedregion-of-interest. This variability in coil sensitivity, as describedherein, can be exploited to characterize location and/or direction ofmotion when acquiring data with a multi-coil or phased-array assembly.

Referring now to FIG. 9, a comparison of k-space origin data 136 withrespiratory bellows data 138 from an axial liver scan in afree-breathing subject is shown. K-space origin data 136 was acquiredwith a single channel of a phased-array coil. As illustrated, thek-space origin data tracks the respiratory bellows data 138 therebyindicating that breathing motion information can be extracted from theMR data at the origin of k-space without having to sample a separaterespiratory bellows signal.

Referring now to FIG. 10, a comparison of k-space origin data 140 withrespiratory bellows data 142 acquired during a coronal liver scan of afree-breathing subject also shows a close correlation between motionencoded in the k-space origin data and the respiratory cycle of thefree-breathing subject. As such, breathing motion information can beextracted from the MR data at the origin of k-space without having tosample a separate respiratory bellows signal.

Referring now to FIG. 11, a comparison of k-space origin data 144 withrespiratory bellows data 146 acquired during a sagittal liver scan of afree-breathing subject also shows a close correlation between motionencoded in the k-space origin data and the respiratory cycle of thefree-breathing subject. As such, breathing motion information can beextracted from the MR data at the origin of k-space without having tosample a separate respiratory bellows signal. Further, FIGS. 9-11illustrate the ability to carry out this extraction independent of aparticular direction of readout.

FIG. 12 illustrates k-space origin data extracted from an axial cardiacscan in the free-breathing subject from which data was acquired forillustration in FIGS. 9-11. High frequency signal fluctuations matchingthe heart rate are visible. The fluctuations are the result of pulsatilecardiac motion in the subject. A slower frequency modulation of the datais also observed which can be attributed to breathing motion of thesubject. In this regard, the present invention may be used tosimultaneously gather both cardiac and respiratory motion informationfrom the k-space origin data during cardiac imaging.

As described herein the present invention determines motion in aregion-of-interest independent of the k-space trajectory employed tosample the region-of-interest. In this regard, the present invention isapplicable with spiral, centric, reverse-centric, center-out, and otherk-space filling schemes. A spiral k-space trajectory is illustrated inFIG. 13, a centric k-space trajectory is illustrated in FIG. 14,reverse-centric k-space trajectory is illustrated in FIG. 15, and acenter-out k-space trajectory is illustrated in FIG. 16.

The present invention has been described with respect to the extractionof motion information within a region-of-interest directly from rawk-space data without implementation of separate physiological motionmeasuring systems. Moreover, by employing a direct extraction of motioninformation from k-space, the present invention provides a measurementof motion in the region-of-interest that is more accurate when comparedto indirect techniques of assessing motion, i.e. ECGs. Additionally, thepresent invention is capable of ascertaining magnitude as well as phaseinformation with respect to motion in a region-of-interest. In thisregard, the present invention may be applied to not only improvingsynchronization of MR data in a gated acquisition, but also used inslice tracking and phase error correction. Furthermore, not only is thepresent invention k-space trajectory independent, it may also beimplemented to assess multiple types of motion in a region-of-interest,such as respiratory, cardiac, and other anatomical induced motion.Moreover, the present invention is applicable with single coil andmulti-coil receivers and, as such, can be used to differentiate locationand direction of motion in an excited region based on coil sensitivityto the excited region. An understanding of location and direction ofmotion can further enable accurate assessment of motion in theregion-of-interest for acquisition synchronization, slice tracking, anderror correction.

The present invention is particularly applicable to body imaging scanswhere respiratory motion can severely limit image quality, especially insubjects who cannot hold their breath for extended periods of time. Thepresent invention may also be applicable in cardiac imaging. In thisregard, a separate ECG acquisition can be avoided and motion informationderived directly from MR data can be used as a cardiac gating triggerfor CINE imaging and other cardiac protocols. Detected respiratorymotion of the heart could also be used to synchronize cardiac imagingwith breathing or to perform slice tracking, particularly in coronaryartery imaging applications, which are vulnerable to breathing motion.

Therefore, an MRI apparatus is disclosed and includes a magneticresonance imaging (MRI) system having a plurality of gradient coilspositioned about a bore of a magnet to impress a polarizing magneticfield and an RF transceiver system and an RF switch controlled by apulse module to transmit RF signals to an RF coil assembly to acquire MRimages. A computer is programmed to sample MR signals from aregion-of-interest having motion therein with a given k-space trajectoryto fill k-space and determine motion in the region-of-interest directlyfrom sampled MR data acquired from the MR signals independent of thegiven k-space trajectory used to fill k-space.

The present invention also includes a method of MR imaging. The methodincludes sampling MR data over a plurality of repetition time intervalsfor a central region of k-space filled, using a given k-space fillingtrajectory, with MR data acquired from the region-of-interest havingmotion therein. The method also includes the steps of monitoringmotion-induced modulation in the MR data for the central region ofk-space over the plurality of repetition time intervals and determiningmotion in the region-of-interest from the motion-induced modulationindependent of the given k-space filling trajectory.

The present invention may also be found in a computer readable storagemedium having a computer program stored thereon to assess motion in aregion-of-interest. The computer program includes a set of instructionsthat when executed by a computer causes the computer to sample a centralregion of k-space each repetition time interval of a pulse sequenceapplied to acquire MR data from the region-of-interest. The computer isthen caused to measure modulation of MR data in the central region overseveral repetition time intervals. The computer is also caused todetermine motion in the region-of-interest based on differences inmagnitude and phase measured in the MR data over the several repetitiontime intervals independent of k-space trajectory used to sample theregion-of-interest.

The present invention further includes a method of MR imaging thatincludes acquiring a first set of non-spatially encoded MR data from aregion-of-interest prior to application of spatially encoding gradientsand acquiring a second set of non-spatially encoded MR data from theregion-of-interest after application of rewinder gradients. Motion inthe region-of-interest is then determined from the first and the secondset of non-spatially encoded MR data.

The present invention has been described in terms of the preferredembodiment, and it is recognized that equivalents, alternatives, andmodifications, aside from those expressly stated, are possible andwithin the scope of the appending claims.

1. A magnetic resonance (MR) imaging apparatus comprising: an MR imagingsystem having a plurality of gradient coils positioned about a bore of amagnet to impress a polarizing magnetic field and an RF transceiversystem and an RF switch controlled by a pulse module to transmit RFsignals to an RF coil assembly to acquire MR images; and a computerprogrammed to: sample non-spatially-encoded MR data of a central regionof k-space from a region-of-interest (ROI) using any given k-spacetrajectory, wherein the non-spatially-encoded MR data is sampled duringa repetition time interval of a pulse sequence; sample spatially-encodedMR data from the ROI during the repetition time interval, wherein thespatially-encoded MR data comprises MR imaging data; sample additionalnon-spatially-encoded MR data of the central region of k-space from theROI; and determine motion in the ROI based on MR motion data comprisingthe non-spatially-encoded MR data and the additionalnon-spatially-encoded MR data, wherein the MR motion data is free of MRimaging data.
 2. The MR imaging apparatus of claim 1 wherein the centralregion of k-space comprises the center of k-space and a plurality ofk-space points about the center of k-space.
 3. The MR imaging apparatusof claim 1 wherein the computer is further programmed to: determine amagnitude modulation among the non-spatially-encoded MR data and theadditional non-spatially-encoded MR data; and determine a gating signalbased on the determination of the magnitude modulation to at least oneof prospectively and retrospectively trigger a gated acquisition.
 4. TheMR imaging apparatus of claim 1 wherein the additionalnon-spatially-encoded MR data is sampled during the repetition timeinterval.
 5. The MR imaging apparatus of claim 4 wherein thenon-spatially-encoded MR data is sampled during the repetition timeinterval prior to the onset of a spatial encoding gradient to sample thespatially-encoded MR data, and wherein the additionalnon-spatially-encoded MR data is sampled after application of a rewinderpulse.
 6. The MR imaging apparatus of claim 5 wherein the gatedacquisition is at least one of a cardiac gated acquisition scheme and arespiratory gated acquisition scheme.
 7. The MR imaging apparatus ofclaim 5 wherein the computer is further programmed to determine amodulation among the non-spatially-encoded MR data and the additionalnon-spatially-encoded MR data for the determination of the motion,wherein the modulation is at least one of a phase modulation and amagnitude modulation.
 8. The MR imaging apparatus of claim 1 wherein thecomputer is further programmed to sample the additionalnon-spatially-encoded MR data during a repetition time intervalsubsequent to the repetition time interval in which thenon-spatially-encoded MR data is sampled.
 9. The MR imaging apparatus ofclaim 1 wherein the computer is further programmed to: select an RF coilselected from the RF coil assembly that is most sensitive to the motion;and sample the non-spatially-encoded MR data and the additionalnon-spatially-encoded MR data with only the selected RF coil.
 10. The MRimaging apparatus of claim 1 wherein the non-spatially-encoded MR data,the additional non-spatially-encoded MR data, and the spatially-encodedMR data are sampled without subjecting a subject to breath-holding. 11.A computer readable storage medium having a computer program storedthereon, the computer program comprising a set of instructions that whenexecuted by a computer cause the computer to: acquire a first and asecond set of central k-space magnetic resonance (MR) data from aregion-of-interest (ROI) at a time other than during an application offrequency and phase encoding gradients, wherein at least the first setof central k-space MR data is acquired during a first repetition time ofa pulse sequence; acquire MR imaging data from the ROI during the firstrepetition time and during application of at least one of the frequencyand phase encoding gradients; assess motion in the ROI based on thefirst and second set of central k-space MR data, wherein the assessmentof motion in the ROI is independent of the MR imaging data; andreconstruct an MR image based on the assessment of motion.
 12. Thecomputer readable storage medium of claim 11 wherein the set ofinstructions cause the computer to acquire the second set of centralk-space MR data during the first repetition time.
 13. The computerreadable storage medium of claim 11 wherein the set of instructionsfurther cause the computer to correct at least one phase error in the MRimaging data based on the assessment of motion before reconstruction ofthe MR image.
 14. The computer readable storage medium of claim 11wherein the instructions that cause the computer to assess motion causethe computer to determine a DC fluctuation of at least one of a phaseand magnitude among the first and second sets of central k-space data;and wherein the instructions further cause the computer to adjust aslice position based on the DC fluctuation.
 15. A method of magneticresonance (MR) imaging comprising: sampling a central region of k-spaceassociated with a region-of-interest (ROI) during a first repetitioninterval of a pulse sequence defined by an RF pulse using any k-spacetrajectory, wherein the sampling of the central region of k-space occursprior to any application of spatial-encoding gradients during therepetition time interval such that a first non-spatially encoded dataset is obtained; sampling spatially-encoded MR data from the ROI duringthe first repetition interval; determining motion in the ROI based on MRmotion data, wherein the MR motion data comprises the firstnon-spatially encoded data set from the central region of k-space, andwherein the MR motion data is free of spatially-encoded MR data; andreconstructing an MR image based on the determination of motion in theROI.
 16. The method of claim 15 further comprising: sampling the centralregion of k-space associated with the ROI during the first repetitioninterval using the any given k-space trajectory to obtain a secondnon-spatially encoded data set; and wherein determining the motion inthe ROI comprises assessing at least one of magnitude and phasefluctuations among the first non-spatially encoded data set and thesecond non-spatially encoded data set, wherein the MR motion datafurther comprises the second non-spatially-encoded data set.
 17. Themethod of claim 15 further comprising sampling the central region ofk-space associated with the ROI during a second repetition intervalusing the any given k-space trajectory to obtain a second non-spatiallyencoded data set, wherein the MR motion data further comprises thesecond non-spatially encoded data set.
 18. The method of claim 17wherein determining the motion in the ROI comprises assessing at leastone of magnitude and phase fluctuations among the first non-spatiallyencoded data set and the second non-spatially encoded data set, whereinthe MR motion data further comprises the second non-spatially-encodeddata set.
 19. The method of claim 15 further comprising realigning thespatially-encoded MR data prior to reconstructing the MR image.
 20. Themethod of claim 15 further comprising gating additional samplings ofspatially-encoded MR data based on magnitude fluctuations, whereingating additional samplings comprises at least one of cardiac gating andrespiratory gating.